Methods for providing high beam quality for planar lasers in the relatively small cross section (SCS) size direction are well known. Such lasers are known as waveguide lasers when their LCS dimension (viewed in a cross section through the laser) is relatively large and their SCS dimension is relatively small and typically less than 200 μm. Such lasers are known as slab lasers when their LCS dimension (viewed in a cross section through the laser) is again relatively large, but their SCS dimension is thicker than a waveguide laser, perhaps in the range of 0.5 to 1 mm. In either case they are usually based on a proper distribution of the refractive index along the SCS size direction or on matching the fundamental mode size to the gain size in the SCS size direction. A considerable problem arises, however, with mode selection in the LCS size direction. This proposed method overcomes this obstacle through the application of at least one inclined reflecting or transmitting thick Bragg grating (BG) in the optical track of the laser resonator. The inclination of the grating suppresses the higher-order transversal modes in the LCS size direction because of a very narrow angular selectivity of the BGs in the plane of beam diffraction. Therefore, only the lowest spatial mode oscillation will be available in such resonators. This process will be facilitated because the thick BGs operate as apodized apertures for reflected or transmitted beams. In addition, the application of BGs allows for the stabilization of the laser wavelength.
Currently, high brightness lasers or high-energy lasers (HEL) with a small beam divergence are needed in many areas including free space communication, precise material processing, military applications, etc. Planar active elements like waveguides and thin slabs are very promising structures for the HEL design in comparison with the traditional rod elements because they have:                high-gain performance, which is a common property of all waveguides;        considerably higher thermal-stress fracture limit (see J. M. Eggleston, T. J. Kane, K. Kuhn, J. Unternahrer, R. L. Byer. “The slab geometry laser—Part I: Theory,” IEEE J. Quantum Electron. v. 20, pp. 289-301, 1984);        relatively low birefringence because of one-dimensional heat flow in the slab (see Eggleston, infra);        good geometry for heat removal and pumping with a stripe output of high-power diode bars.        
Therefore, much research devoted to the development and optimization of thin slab and waveguide lasers has been carried out over the last few years. See, for example:                1. C. L. Bonner, C. T. A. Brown, D. P. Shepherd, A. C. Tropper, D. C. Hanna. “Diode-bar end-pumped high-power Nd:Y3Al5O12 planar waveguide laser,” Opt Lett., v. 23, pp. 942-944, 1998.        2. A. Faulstich, H. J. Baker, D. R. Hall, “Face pumping of thin, solid-state slab lasers with laser diodes” Opt. Lett., v. 21, pp. 594-596, 1996        3. H. J. Baker, A. A. Chesworth, D. Pelaez-Millas, D. R. Hall. “A planar waveguide Nd:YAG laser with a hybrid waveguide-unstable resonator,” Opt. Commun. v. 191, pp. 125-131, 2001.        4. J. R. Lee, H. J. Baker, G. J. Friel, G. J. Hilton, D. R. Hall “High-average-power Nd:YAG planar waveguide laser that is face pumped by 10 laser diode bars,” Opt Lett., v. 27, pp. 524-526, 2002        5. K. Du, N. Wu, J. Xu, J. Giesekus, P. Loosen, R. Poprawe. “Partially end-pumped Nd:YAG slab laser with a hybrid resonator,” Opt. Lett., v. 23, pp. 370-372, 1998.        6. K. Du, D. Li, H. Zhang, P. Shi, X. Wei, R. Diart. “Electro-optically Q-switched Nd:YVO4 slab laser with a high repetition rate and a short pulse width,” Opt. Lett., v. 28, pp. 87-89, 2003.        
The basic disadvantage of planar lasers is a very large beam divergence in the LCS size direction because of the rather large size of the active elements in this direction, especially in the case of lasers with plane resonators (See documents 1-4 mentioned above). To improve the beam divergence, different hybrid resonators with cylindrical mirrors were used (See documents 3-6 mentioned above). Such mirrors produced a stable resonator in the SCS size direction and an unstable resonator in the perpendicular LCS size direction. This design allowed a considerable improvement in the beam divergence; however, the oscillators were still far from the lowest mode generation.
This disclosure addresses this problem through the application of at least one inclined reflecting or transmitting thick BG in the optical track of the laser resonator. The BG produced (recorded) in the waveguide or in the thin slab is preferably perfectly matched with the planar shape to provide a strong angular selectivity exactly in the required LCS size direction.
In accordance with the well-known Kogelnik's coupled wave theory the widest angular selectivity of a BG is reached when the angle of beam incidence is close to normal. This value is strongly decreased with an increase in the angle of incidence and the thickness of the grating. Therefore, it is possible by the proper selection of these parameters to satisfy any reasonable requirements for beam divergence restrictions. The additional advantage of the inclined BG use in our case arises from the fact that the inclined grating diffraction efficiency falls smoothly to zero closer to the beam borders, providing a spatial apodization of the beam. This circumstance facilitates the lowest mode oscillation in the considered direction.
Thus, the basic advantage of the disclosed techniques over the existing art for forming high-quality beams from planar lasers is the opportunity to adjust the angular selectivity of the inclined BG in accordance with the planar laser design so as to provide the oscillation of the lowest spatial mode in the LCS size direction.
The currently disclosed method and apparatus will provide a high brightness of the lasers in combination with a narrow band spectrum of radiation. Such lasers are expected to be widely used in communication networks and in high-energy laser systems as master oscillators.
A hybrid resonator consisting of two cylindrical mirrors with radiuses of 500 mm and 415 mm is disclosed by Documents 3 and 4 mentioned above to improve the beam divergence in the LCS size direction. By the optimization of the distance between the mirrors and the ends of the waveguide element, the authors were able to decrease the beam divergence considerably and to reach the M2 magnitude of ˜1.5 at 5 W of power in the beam, where M2 is a standard value for beam quality characterization. However, increasing the pump power to 250 W resulted in a degradation of the beam quality to M2=2.3 and then, to M2=4.4 at elevated pump power. Moreover, the change in the pump power resulted in defocusing the unstable resonator and required mirror realignment to optimize the beam quality and output power of the laser.
Another hybrid resonator has been described in Document 5 mentioned above. To get high-power oscillation with high beam quality, the authors used two highly reflective spherical mirrors with radiuses of 122 mm and 115 mm (See FIG. 1). The edge mirror (M3) was used for off-axis output coupling in the unstable plane (in the LCS size direction). The optimization of the scheme for definite pump power (74 W) allowed reaching an M2 magnitude of ˜1.3 at 31 W of output power. As noted previously, any change in the pump power should result in realignment of the resonator.
Finally, one more type of hybrid resonator was applied to design a Q-switched slab laser (see Document 6 mentioned above) with an output power of 103 W and beam quality M2<1.5. The scheme of this resonator is shown in FIG. 2.
All the above-mentioned schemes have the same disadvantages resulting from a very high sensitivity of unstable resonators to the thermal lens in the planar element. Therefore, any change in the pump power requires realignment of the resonators and such an approach did not provide high beam quality for high-power oscillation.
The application of BGs to planar waveguide lasers (see FIG. 3) is disclosed by Y. Y. Lin et al. (see Y. Y. Lin, J. T. Shy, A. C. Chiang, Y. C. Huang. “Optical parametric oscillator with distributed feedback grating of distributed Bragg reflector,” US Patent Published Application 2003/0123497, Jul. 3, 2003). However, in that disclosure, the gratings are used for spectral selection and are operated under a normal angle of beam incidence. Therefore, the gratings have the widest possible angular distribution and do not work as angular selectors.
The present disclosure provides a method for spatial mode selection of a planar waveguide or thin slab laser in the laser's large cross section size direction, the method comprising:                (i) providing a planar waveguide or thin slab laser, the planar waveguide or thin slab laser having a small cross section size direction orthogonal to a major axis thereof and a large cross section size direction orthogonal to the small cross section size direction and to the major axis;        (ii) disposing a planar reflector at one end of the planar waveguide or thin slab laser resonator, the planar reflector having at least one reflecting surface which is arranged orthogonally to the major axis and parallel to both the small and large cross section size directions of the laser; and        (iii) disposing a Bragg grating in the optical track of the laser resonator, the Bragg grating having a grating vector which is arranged in the plane normal to said small cross section size direction and at the angle between 0° and 90° to said large cross section size direction for suppression of higher order optical modes in said large cross section size direction.        
The present disclosure also provides a planar waveguide or thin slab laser comprising:                a planar waveguide or thin slab active medium, the planar waveguide or thin slab active medium having a small cross section size direction orthogonal to a major axis thereof and a large cross section size direction orthogonal to the small cross section size direction and to the major axis;        a planar reflector disposed at one end of the planar waveguide or thin slab laser resonator, the planar reflector having at least one reflecting surface which is arranged orthogonally to the major axis and parallel to both the small and large cross section size directions of the active medium; and        a Bragg grating disposed in the optical track of the laser resonator, the Bragg grating having a grating vector which is arranged in the plane normal to said small cross section size direction and at the angle between 0° and 90° to said large cross section size direction for suppression of higher order optical modes in said large cross section size direction.        